This invention is concerned with the process for the aromatization of a cyclic compound to form a fully aromatic ring structure. More particularly, it relates to a process for producing a halogen substituted phthalic anhydride from a halogen substituted tetrahydrophthalic anhydride or from a dihalogen substituted hexahydrophthalic anhydride.
Substituted phthalic anhydrides are valuable raw materials for the synthesis of useful products. These anhydrides are utilized as intermediates in the synthesis of organic polymers, dyes, plasticizers and in other uses.
Procedures are known for preparing substituted phthalic anhydrides. U.S. Pat. No. 4,560,772 discloses the reaction of 4-methyltetrahydrophthalic anhydride with excess sulfur and a catalytic amount of zinc oxide and 2-mercaptobenzothiazole to produce 4-methylphthalic anhydride and hydrogen sulfide.
U.S. Pat. No. 4,560,773 discloses a similar reaction between the electron rich 4-methyl-tetrahydrophthalic anhydride and bromine in the presence of a catalytic amount of an acid acceptor such as dimethylformamide or pyridine in the liquid phase.
U.S. Pat. No. 4,709,056 discloses the dehydrohalogenation of dihalohexahydrophthalic anhydrides through the use of a basic alumina catalyst in a liquid phase to produce 4-fluoro-1,2,3,6-tetrahydrophthalic anhydride.
Ohkatsu et al., J. Japan Petrol. Inst., 22, 164-9 (1979) discloses the dehydrogenation of hydrocarbons using an activated carbon bed to produce the corresponding olefins. The mechanism of the reaction using cyclohexane and cyclohexene were studied using a pressure flow technique.
Bergmann J. Amer. Chem. Soc. 64, 176 (1942) discloses the aromatization of tetrahydrophthalic anhydride products of Diels-Alder reactions. The author discloses that dehydrogenation occurs when the tetrahydrophthalic anhydride product is boiled in nitrobenzene. However, it is further disclosed that dehydrogenation does not occur when p-bromonitrobenzene, p-chloronitrobenzene, or m-dinitrobenzene in xylene is employed. Moreover, it has been found that when the dihalohexahydrophthalic anhydrides of this publication are dehydrogenated in nitrobenzene, a portion of the nitrobenzene is reduced to aniline. The aniline reacts with the anhydride group of either the starting material or product to form imides and thus lower the yield of desired product.
Skvarchenko et al, Obshchei Khimiil, Vol. 30, No. 11, pp 3535-3541 (1960) disclose the aromatization of chlorosubstituted tetrahydrophthalic anhydride by heating with phosphorus pentoxide. In the aromatization process described however, decarboxylation also occurs with the formation of the corresponding chlorosubstituted benzene compound. The preparation of tetrahydrophthalic acids and anhydrides and various methods for dehydrogenation and aromatization thereof are reviewed by Skvarchenko in Russian Chemical Reviews. Nov. 1963, pp. 571-589.
The aromatization of organic compounds has been shown by various techniques and the use of special catalysts seem to control the satisfactory conversion at useful product yields and conversion times. This study was undertaken to determine if a vapor state aromatization of halogen substituted materials such as halogenated tetrahydrophthalic anhydrides or geminal dihalogen hexahydrophthalic anhydrides could be converted to a monohalogenated phthalic anhydride at adequate yields and within a reasonable time.
Experimentation has shown that the use of air flow and a heated activated carbon surface can aromatize halogenated saturated phthalic anhydrides to halogenated aromatic phthalic anhydrides, and is the subject of this application.
It is the object of the present invention to provide novel intermediate compounds which are useful and provide a commercially attractive synthetic route.